(CNN) -- A man was killed by a grizzly bear Wednesday morning at Yellowstone National Park, according to the National Park Service.

The visitor was hiking with his wife along the Wapiti Lake trail when he surprised a female bear with her cubs, the park service said. The bear attacked and fatally wounded the man in an attempt to defend her cubs. A group of nearby hikers called police after they heard the victim's wife crying for help.

The name of the victim is being withheld pending notification of family members.

There have been no reports of bear encounters along or near the Wapiti Lake trail this season, park spokesman Al Nash said in the statement.

A bear warning sign was posted at the head of the trail because of known bear activity along access points.

All trails and back-country campsites have been closed until further notice. Park rangers are working to clear the area of all back-country users.

"It is extremely unfortunate that this couple's trip into the Yellowstone backcountry has ended in tragedy," said Dan Wenk, superintendent of Yellowstone National Park. "Our heart goes out to the family and friends of the victim as they work to cope with their loss."

This is the first bear-caused human fatality in Yellowstone since 1986, Nash said. Bear attacks are extremely rare. No visitors were injured by bears in the park in 2010.

Park visitors are advised to hike in groups of three or more people and to be alert for bears and make noise in blind spots. It is not known if the victim or his wife was carrying bear pepper spray at the time of the incident. Bear pepper spray has been effective in stopping aggressive bears.